Manufacture of articles from fly ash

ABSTRACT

Methods of forming a shaped article having a matrix that contains sintered fly ash are disclosed that include forming a fly ash dough that includes fly ash and water. In one form a superplasticiser is added in the dough. A green article is formed in a desired shape from the fly ash dough that is subsequently fired so that the shaped article is hardened by sintering its fly ash matrix. In one form, the green article is cured under conditions of moderate heating and high humidity. A building element having a matrix of sintered fly ash is also disclosed.

RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.10/721,581, filed Nov. 24, 2003, which is a continuation-in-part of U.S.patent application Ser. No. 09/996,528, filed Nov. 28, 2001, whichclaims priority on Australian Provisional Patent Application No.2003905445 dated Oct. 3, 2003.

FIELD OF THE INVENTION

The present invention relates generally to shaped articles that areformed from fly ash and to methods of forming such articles. Theinvention has been developed especially, but not exclusively, for themanufacture of structural elements and the invention is herein describedin that context. However it is to be appreciated that the invention hasbroader application and may be used for the production of a vast rangeof articles, both structural and non structural.

BACKGROUND OF THE INVENTION

Fly ash is a by-product from the burning of coal in coal fired powerstations. Fly ash is made in abundance and typically contains heavymetals such as cadmium, chromium, zinc and lead that make disposalproblematic. In trying to minimise the environmental impact of fly ash,various uses of fly ash have been contemplated to both aid in fly ashdisposal and to obtain some economic return.

One such use is in the manufacture of bricks that contain fly ash as aconstituent part. These bricks usually include fly ash blended with clayand are fire hardened. Whilst these bricks find a use for fly ash, theyhave not been seen as a viable structural building element. Inparticular, difficulties have been encountered in manufacturing brickscontaining fly ash that are cost competitive with existing bricks, areof a consistent quality, and perform adequately over a range ofstructural properties.

SUMMARY OF THE INVENTION

In a first aspect, there is provided a method of forming a shapedarticle having a matrix containing sintered fly ash, the methodcomprising the steps of:

-   -   blending fly ash together with water to produce a fly ash dough;    -   forming a green article in a desired shape from the fly ash        dough;    -   curing the green article to at least partially solidify the        article at between 30-80° C. and 20%-60% relative humidity; and    -   firing the green article so that the shaped article is hardened        by sintering its fly ash matrix.

In the above method, the green article is cured before it is fired.During curing, the water reacts with the fly ash so as to solidify thearticle.

The solidification of the green article during this curing process maybe contributable to several different reactions. Whilst not binding theinvention to theory, the inventors consider that where the fly ash isthe only cementitious material in the matrix, the only compound that cangive certain quick solidification is the calcium oxide. This compound isavailable in small quantity in class F fly ash and in much largerquantities in class C fly ash. The reaction between water and calciumoxide results in the formation of calcium hydroxide which lends somesolidification to the article. Subsequently, a pozzolanic reactionoccurs where the main oxides in the fly ash, primarily the silica andthe alumina, react with the calcium hydroxide to form a much harder andmore cementing material than the hydroxide. The resulting material is acomplex crystalline and amorphous mixture of products that contain intheir lattice molecules of silicon oxides, aluminium oxides, calciumoxides and water.

In a particular embodiment, any free water in the dough is reduced fromthe matrix whilst the green article is cured. In the above form, thegreen article is subjected to low to moderate heating during this curingprocess. The advantage of this arrangement is that the gentle heatingcan reduce the free water without causing undue cracking of the matrix.Also, the slow withdrawal of water still gives time for some of thewater to react with the fly ash both by hydrating the cementitiousmaterial in the fly ash and under the pozzolanic reaction. In one form,the green article is heated under elevated humidity. The advantage ofthis arrangement is that it can promote the solidifying of the greenarticle more evenly throughout the article.

This curing process consumes free water that is already in the dough andmay need some additional water to compensate for self desiccation. Inthe above form the additional water can be drawn from the humidatmosphere.

In the process according to the above form, use is made of two separatereactions; first by gaining initial solidification through the formationof calcium hydroxide, and second by gaining further solidificationthrough the pozzolanic reaction. If the process only relied on theformer of these reactions, the solidification of the green articlebefore firing would be limited due to the limited amount of calciumoxide in the fly ash. The advantage of solidifying the green article isthat it improves its capacity to be handled, and its dimensionalstability during firing, both of which are important in commercialmanufacture of the shaped article.

In one form, the green article is cured in a temperature in the range of30° C. to 80° C. In one form, the green article is cured in atemperature the range of 55° C. to 65° C.

In one form, the green article is subjected to conditions where thehumidity is in the range of 20% relative humidity to 60% relativehumidity. In one form, the green article is subjected to conditionswhere the humidity is in the range of 35% relative humidity to 45%relative humidity.

When done under gentle or moderate heat and high humidity, the durationof curing may vary considerably as extended curing time is unlikely tocause the matrix of green article to crack. Typically, for bricks, thecuring time will be in the order of 12 hours to 5 days, and morepreferably between 1 and 3 days. Whilst the curing is important toremove water and to solidify the green article, it is desirable toreduce the curing time to minimise the manufacturing process time. Inone form, the inventors have found 2 days sufficient for curing.

In one form, e water is added in excess of that which is absorbed by thefly ash so that the dough contains free water so as to be in at least apartially fluid state. In one form, at least a portion of the free waterfrom the fly ash dough is removed during and/or after forming of thegreen article.

In one form, the majority, if not all, of the free water is removedprior to firing of the green article. As such, the porosity of the firedarticle can be better controlled as the firing process will not generatecracking or bursting as a result of water vaporising in the matrix.

In one form, the moisture content remaining in the green article priorto firing is in the range of 1% to 5%. In one form, the moisture contentremaining in the green article prior to firing is in the range of 2% to4%. Typically the moisture remaining in the article is made up of twocomponents. The first is the moisture entered into the hydrationreaction and produced solid products of calcium silicate and aluminiumsilicate hydrate complexes. The second part is that which is trapped asmoisture within the internal pores. The first component resistscrumbling of the brick during handling and to withstand internalpressures of the escaping gases during firing. The second component is amain source of porosity that remains in the brick structure.

In a particular embodiment, a superplasticiser is blended with the flyash and water. The advantage of using a superplasticiser is that itreduces the amount of free water that is required to make the dough in aworkable state. This in turn alleviates the amount of water that mayneed to be subsequently removed to achieve the desired properties in thearticle, thereby allowing for more efficient processing of the articleand also allowing for better control over the shape and size of thearticle during its production.

In another aspect, there is provided a method of forming a shapedarticle having a matrix containing sintered fly ash, the methodcomprising the steps of:

-   -   forming a fly ash dough incorporating fly ash, water and a        superplasticiser;    -   forming a green article in a desired shape from the fly ash        dough; and    -   firing the green article so that the shaped article is hardened        by sintering its fly ash matrix.

In one form, the method merely uses three ingredients, namely fly ash,water and a plasticiser. As fly ash is a by-product, it is aninexpensive and readily available constituent. Further, the method canbe used in a production line fashion, akin to clay brick manufacture. Bycontrolling the water content in the dough, the articles may beinitially shaped without the need of a mould as the dough may exhibitadequate dimensional stability. Also, the properties of the article canbe readily controlled by controlling the water content of the greenarticle, and the firing temperature and duration. Each of theseparameters can be controlled during manufacture thereby allowing forarticles to be produced of consistent quality.

In any form described above, it is to be appreciated that otheradditives may be incorporated into the mixture if required. For examplepigments may be incorporated to impart certain coloration to thearticle. Also further additives may be incorporated to improve theproperties of the mixture or resulting in green article. For example,quantities of carboxymethyl cellulose (CMC) may be incorporated inminute quantities to gel the mixture without the need of excessivewater. Such additives also protect the dough from potential shrinkage,and cracking in the case of prolonged curing periods. Similar effects tothat of CMC may also be obtained from the addition of minute quantitiesof calcium chloride solution.

The methods disclosed above have particular application for themanufacture of structural elements such as bricks. The inventors havefound that bricks formed solely, or at least principally, from sinteredfly ash have a higher compressive strength and modulus of rupture thanconventional clay bricks. Also, by controlling the water content in thegreen article and the firing temperature and duration, it is possible tocontrol the structure of the fly ash matrix and its surfacecharacteristics. This in turn allows for the initial rate of absorptionand absorption capacity of the article to be controlled, both of whichare important properties, particularly in brick manufacture. Further,reducing the free water reduces the risk of bursting when the greenarticle is fired and thus provides for a more uniform sintering processthat is as free as possible from internal and external cracking.

Other techniques, such as pressing or the like of the dough or the greenarticle may be used instead of, or in conjunction with, subjecting thearticle to a controlled environment of heat and humidity, to reduce thewater content.

As indicated above the inventors have found that absorption propertiesof the fired article can be regulated by the temperature and duration offiring, particularly where the free water is substantially removed fromthe green article.

In the arrangement where the shaped articles are bricks, in one form,the firing temperature is in the range of 10000° C. to 13000° C. Inanother form, the firing temperature is in the range of between 11000°C. to 12500° C. The duration of firing may be in the range of 30 minutesto 6 hours. The duration of firing may be in the range of 1 to 4 hours.

The sintered fly ash matrix of bricks fired in this range tends not tobe glazed and exhibit excellent absorption characteristics both in termsof initial rate of absorption and absorption capacity.

In a further aspect, there is provided a building element having amatrix of sintered fly ash and having a compressive strength of greaterthan 30 MPa, a modulus of rupture greater than 5 Mpa, an initial rate ofabsorption (IRA) of between 0.2 to 5 kg/m²/min and an absorptioncapacity of between 5-20%.

Building elements formed with these properties are ideally suited as adirect replacement for conventional clay bricks. They are stronger thanconventional clay bricks, particularly in tension, and are capable ofbonding well with mortar due to their absorption properties. Whilst thestrength of the elements is due to the sintered fly ash matrix, theabsorption properties are due to the porosity of the elements and theirsurface characteristics. As such, the building elements according tothis aspect are ideally suited to be manufactured by the earlier aspectof the invention where the porosity and surface characteristics can becontrolled.

Other features and advantages of the present invention will becomeapparent from the following more detailed description, taken inconjunction with the accompanying drawings, which illustrate, by way ofexample, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

It is convenient to hereinafter describe an embodiment with reference tothe accompanying drawings. It is to be appreciated that theparticularity of the drawings and the related description is to beunderstood as not superseding the preceding broad description ofinventions.

The accompanying drawings illustrate the invention. In such drawings:

FIG. 1 is a photograph of a cross section of a brick having a sinteredfly ash matrix;

FIG. 2 is a flow chart illustrating the steps in manufacturing fly ashbricks;

FIG. 3 is a graph of absorption capacity of a fly ash brick as afunction of firing temperature;

FIG. 4 is a graph of initial rate of absorption of the fly ash brick asa function of firing temperature;

FIG. 5 is a graph of moisture content of bricks as a function of time ofcuring;

FIG. 6 in a micrograph of the fly ash brick matrix when fired at atemperature of 1200° C.; and

FIG. 7 is a micrograph of the fly ash brick matrix when fired at atemperature of 1040° C.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning firstly to FIG. 1, a fly ash brick 10 is disclosed whichincorporates a matrix 11 which is made from sintered fly ash 12 havingvoids 13 dispersed therethrough. The structure of the sintered fly ashand the arrangement and dispersion of the voids dictate the structuralstrength of the brick 10 and its absorption capacity (both the initialrate of absorption as well as the total absorption capacity).

In general, the voids 13 are dispersed throughout the matrix andcomprise predominantly small interconnected voids 14 and larger isolatedvoids 15. The small voids 14 make the brick 10 porous and capable ofabsorbing water. These small voids 14 are largely a function of thepacking density of the fly ash, and the degree of compaction of the flyash dough undertaken in manufacture of the brick. Of equal importance,these voids are a function of the efficiency of the sintering processthat is controlled by the firing temperature and duration. The smallervoids 14 are also partly due to the superplasticiser used in manufacturethe dough. The inclusion of a superplasticiser reduces the amount ofwater required to blend the fly ash dough whilst allowing ease ofworkability. The dispersant effect of the superplasticiser is such thatthe water is held in the form of droplets that allow the fly ashparticles to roll on them and when dried through evaporation and/or selfdesiccation, the droplets leave behind their traces as air bubbles.

Fly ash particles typically have a particle size ranging from 1 μm to150 μm. Typically, more than 66% of fly ash particles have a diametersmaller than 45 μm. The median diameter ranges from 2 μm to 10 μm andthe reactivity of the ash increases with the smaller size particles.While the packing density may be regulated by grading of the fly ash,the inventors have found that no such screening is required to give therequired properties of strength and absorption capacity as detailedbelow. This has the distinct advantage that no pre-treating of the flyash is required. The fly ash can be collected from source (typically apower station) and used directly as a constituent in the brickmanufacturing process.

The fly ash used in the brick of FIG. 1 is Class F. Class F fly ash isproduced from bituminous coal and is mainly silicious. According to ASTMclassification, class F fly ash contains a total of at least 70% of itscompounds being of silicon oxide, aluminium oxide and iron oxide.Another type of fly ash is known as class C fly ash. This is derivedfrom sub-bituminous and lignite coal. Class C fly ash is rich withcalcium oxide. Whilst the typical content of calcium oxide in class Ffly ash is between 2-4% and is generally lower than 10%, the typicalcontent of calcium oxide in class C fly ash is between 10% and 20% andcan be as high as 26%. Whilst bituminous class F fly ash is used in thisembodiment, it should be understood that this disclosure is notrestricted to this type and is applicable to type C fly ash as well.Moreover, the high content of calcium oxide present in the class C flyash serves to accelerate solidification and reduce the curing time andhence reduces the time required for handling and firing processes toproceed.

The larger voids 15 are formed primarily from air entrapped in thematrix when the brick is being formed. These voids 15 are partly afunction of the manufacturing process and in particular the initialmixing of the fly ash and water to form a dough, and the compaction ofthat dough. The superplasticiser through its dispersant and hydrophobiceffect may also contribute to the formation of larger voids in the driedproduct.

Ideally, the matrix 11 does not include an excessive amount of thelarger voids 15 as they weaken the matrix. However, these larger voidscan contribute to the brick properties as they serve to alleviatepossible pressure build-up while firing and serve to alleviate stressesthat may occur in the finished product in places where freezing andthawing are encountered. As the brick 10 was manufactured underlaboratory conditions, there was some restriction on controlling thepresence of larger voids 15. It is anticipated that the generation ofthe larger voids would be better controlled under commercial procedureswhere the formation of the fly ash matrix could be better controlled.

As illustrated in the photograph of FIG. 1, the outer margins 16 of thebrick, adjacent the outer peripheral edge 17, are still porous. Whilstthe brick 10 incorporates a skin 18 formed on firing of the brick it isnot glazed and still incorporates the smaller voids 14. As such the skindoes not form a barrier to water penetration into the brick 10.

Also, there is an absence of major cracks or fissures extending throughthe brick matrix that would significantly reduce the brick strength andpromote inconsistent water absorption of the brick.

The structure of the brick matrix 11 provides consistent strength andwater absorption characteristics that make the brick 10 ideally suitableas a replacement for conventional clay bricks as will be discussed inmore detail below.

FIG. 2 is a flow chart that schematically represents the process 20 ofmanufacturing the brick 10. In a first stage 21, the constituents of thebrick are provided in their appropriate quantities. The constituentscomprise fly ash, water and a plasticiser.

Fly ash was weighed and placed in a suitable concrete mixer or similar.About seventy percent of the total amount of water was then added andthe dough mixture blended and rotated for three minutes. The totalquantity of water to fly ash was 26 litres of water to 100 kg of flyash. The fly ash used in this experiment was a Class F fly ashconforming to ASTM standard. This is available in abundance from powerstations that use coal. However, it will be appreciated that the use ofa particular fly ash is not a necessity although it should conform to alocal quality standard.

A superplasticiser was then added and mixing continued for anotherperiod of three minutes. The superplasticiser was used in order tofacilitate the workability of the fly ash slurry or dough. Thesuperplasticiser was a pure sodium salt of a polynapthalene sulphonatemade by Handy Chemicals and commercially available under the trade nameDISAL. However, it will be apparent that the use of a particularsuperplasticiser is not necessary. It is only important to achieveconsistent workability with minimum amount of water, and the use of asuitable superplasticiser should be satisfactory provided the dosage isrelevant to the particular superplasticiser that is used. In this case,where DISAL was the superplasticiser, the dosage was at the rate of 200ml per 100 kg of fly ash.

The rest of the water was then added and the mixing was continued forthree more minutes when the mixing was complete.

The mixing of the constituents to form the dough occurs at step 22. Atthat time, the dough may be compacted to limit the voids 13(particularly the larger voids 14). The compaction may be done by anysuitable technique and in the experiments carried out by the inventors,the fly ash dough was placed into a tray and compacted or vibrated on avibrating table in a similar manner to concrete placing. The compactionor compression was stopped when the dough mixture started to bleed.However in a production environment, the fly ash dough may typically bemixed and extruded under pressure which would result in compaction ofthe dough.

At step 23, the green bricks are formed. In the experiments conducted,the dough was cut into the green bricks by cutter moulds forced into thedough. These bricks were then removed from the tray. In a commercialscale operation, where the dough is extruded, the brick would beproduced in a manner adopted for clay brick manufacture where the doughwould be fed on a conveyor belt and cut by wire cutters.

At step 24 the individual green articles are cured by being placed in acuring chamber at 58° C. and 37% relative humidity for a period of 48hours. As indicated previously, the curing process is designed tosolidify the green articles and also to draw out the majority of thewater from the fly ash matrix.

FIG. 5 is a graph of the moisture content of the green fly ash brinkduring curing. This graph shows the moisture content from the time ofmixing until the time of firing which is typically between 24 to 72hours after curing. It is evident that under the conditions of curingthe moisture content stabilisers at about 3.5% after 48 hours. The mainloss of moisture occurs within the first 24 hours. This period is themost critical for encouraging solidification and driving out unnecessarymoisture. From two days onwards the remaining moisture is made up of twocomponents. The first is the moisture that enters into the hydrationreaction and produces solid products of calcium silicate and aluminiumsilicate hydrate complexes. The second part is that of which is trappedas moisture within the internal pores. The first component is necessaryto resist crumbling of the brick during handling and to withstandinternal pressure of the escaping gases during firing. The secondcomponent is a main source of porosity that remains in the brickstructure. The cured bricks are then fired at step 25 so as to sinterthe fly ash matrix. In the experiments, the cured articles were placedin a kiln and the temperature was raised to 1200° C. and the bricks werefired for 3.5 hours.

In a final stage of the process, the sintered fly ash bricks were thenallowed to cool down to room temperature as represented at step 26.

Various properties of the fly ash brick were tested and table 1 belowrepresents the properties of the fly ash bricks compared to common claybricks.

Initial Rate of Brick Compressive Modulus of Absorption AbsorptionAverage Type Strength Rupture (IRA) Capacity Density Clay Typical isfrom From less Typical range 5-20% 1800-2000 Bricks 12 to 40 MPa. Than 1MPa between 0.2 kg/m³ Minimum to greater and 5 Kg/m²/min. Accepted bythan 2 MPa. Australian Default Standard: 7 value is 0.8 MPa. MPa.

The tests conducted to determine the above properties were as follows:

Compressive Strength: Performed according to Australian/New ZealandStandard AS/NZS 4456.4:1997, Method 4: Determining Compressive Strengthof Masonry Units.

Modulus of Rupture: Performed twice, one time according toAustralian/New Zealand Standard AS/NZS 4456.15:1997, Method 15:Determining Lateral Modulus of Rupture, and the second time on unitbricks. The reason why this was done is that the Standard methodrequires forming a beam by horizontally bonding three bricks. The glueused was Epirez, an epoxy mortar binder. This method worked very wellwith normal clay bricks because the glue is stronger in tension than theclay bricks and the failure line was through the brick. In the case ofour fly ash bricks, failure occurred through the glue line at 7.2 MPa.This meant that the bricks are stronger than that and the 7.2 MPa is thestrength of the glue. Hence the testing was done again on single bricksthat involved no glue. The result confirmed that the value of themodulus of rupture fro the fly ash bricks is higher than 7.2 value andis in fact 10.3 MPa.

Initial Rate of Absorption: Performed according to Australian/NewZealand Standard AS/NZS 4456.17:1997, Method 17: Determining InitialRate of Absorption (Suction).

Absorption Capacity: Performed according to Australian/New ZealandStandard AS/NZS 4456.14:1997, Method 14, Determining Water AbsorptionProperties.

Average Density: Performed according to Australian/New Zealand StandardAS/NZS 4456.8:1997 Method 8: Determining Moisture Content and DryDensity.

Accordingly, from the above table, it is clear that the fly ash bricks10 exhibits excellent properties compared to conventional clay bricks.

Two important properties of building bricks are the initial rate ofabsorption (IRA) and the absorption capacity. These two properties areof particular importance for bricks. The IRA is of great importance forthe laying of the bricks and bonding with the mortar. A high IRA resultsin too quick drying of the mortar and thus weakens the mortar andreduces its adherence to the brick. On the other hand if the IRA is toolow, the surface of the brick adjacent to the mortar would not absorbthe excess water and would result in very weak layer of the mortar thatwould not have penetrated enough into the surface crevices and pores ofthe brick. The property of total absorption capacity is also veryimportant for the performance of the brick. A very high absorptionresults in vulnerability to volume changes that would result in crackingof the bricks and structural damage in buildings. It also would lead tocracking in the event of freezing and thawing of the water inside thepores. Too little absorption however is also not desired. This isbecause rain water, rather than get partially absorbed by the brick,would tend to run off very quickly towards the joints and may find itsway into the building as well as reduce the durability of the mortarjoints.

Further tests were conducted by the inventors on the effects of thefiring temperature on the total absorption capacity and the initial rateof absorption. These tests were conducted using green bricks made inaccordance with the above procedure. The only difference being thefiring temperature used. The results of these tests are illustrated inFIGS. 3 and 4.

As is clearly apparent from the FIGS. 3 and 4 is that the temperature offiring has a major effect on the absorption properties of the sinteredfly ash bricks. Further, as can be seen from the above results, bymaintaining the temperature rate between 1100° C. to 1250° C., it ispossible to obtain excellent absorption properties consistent withconventional clay bricks.

FIGS. 6 and 7 are micrographs of the fly ash brick matrix when fired atdifferent temperatures. FIG. 6 has a firing temperature of 1200° C.whereas FIG. 7 is the brick matrix when fired at a temperature of 1040°C. In the micrograph of FIG. 6 the matrix of the brick exhibits finerand more consistent pores throughout the matrix structure. The fly ashis substantially sintered without being glassified. In contrast, in thematrix disclosed in FIG. 7, where the brick was fired at 1040° C., thefly ash is not sintered enough thereby leading to excessive porosity anda reduction in its structural strength.

Accordingly, the invention provides methods of manufacturing articlesfrom fly ash which can be produced on a commercial scale and whichexhibit excellent properties both in terms of strength and absorptioncapacity which makes such articles ideally suited as a substitute forconventional clay bricks.

Although several embodiments have been described in some detail forpurposes of illustration, various modifications may be made withoutdeparting from the scope and spirit of the invention. Accordingly, theinvention is not to be limited, except as by the appended claims.

1. A method of forming a shaped article having a matrix containingsintered fly ash, said method comprising the steps of: blending fly ashtogether with water to produce a fly ash dough; forming a green articlein a desired shape from the fly ash dough; and firing the green articleso that the shaped article is hardened by sintering its fly ash matrix,wherein the green article is fired having a moisture content in therange of 1 to 5%.
 2. (canceled)
 3. A method according to claim 1,wherein the green article is fired having a moisture content in therange of 2 to 4%.
 4. A method according to claim 1, wherein the water isadded in excess of that which is absorbed by the fly ash so that thedough contains free water so as to be in at least a partially fluidstate; and removing at least a portion of the free water from the flyash dough during and/or after forming of the green article.
 5. A methodaccording to claim 4, further comprising the step of curing the greenarticle before it is fired; wherein during the curing of the greenarticle, at least a portion of the free water is removed from the flyash dough.
 6. A method according to claim 7 wherein a superplasticiseris included with the fly ash and water in the fly ash dough.
 7. A methodaccording to claim 5, wherein the green article is cured at atemperature in the range of 55° C. to 65° C.
 8. (canceled)
 9. A methodaccording to claim 5, wherein the green article is subjected to elevatedhumidity during curing.
 10. A method according to claim 9, wherein thehumidity is in the range of 20% relative humidity to 60% relativehumidity.
 11. A method according to claim 10, wherein the humidity is inthe range of 35% relative humidity to 45% relative humidity.
 12. Amethod according to claim 1, wherein the firing temperature is in therange of 1000° C. to 1300° C.
 13. A method according to claim 1, whereinthe firing temperature is between 1100° C. and 1250° C.
 14. A method offorming a shaped article having a matrix containing sintered fly ash,said method comprising the steps of: forming a mixture incorporatingprincipally class F fly ash, water and a superplasticiser; forming agreen article in a desired shape from the fly ash dough; and firing thegreen article so that the shaped article is hardened by sintering itsfly ash matrix.
 15. A method according to claim 14, further comprisingthe step of curing the green article before it is fired, wherein duringcuring the water reacts with the fly ash so as to at least partiallysolidify the article.
 16. A method according to claim 14, wherein thegreen article is cured at a temperature of between 30-80° C.
 17. Amethod according to claim 15, wherein the green article is cured at atemperature of between 55-65° C.
 18. A method according to claim 15,wherein the green article is cured in conditions having between 20%relative humidity and 60% relative humidity.
 19. A method according toclaim 15, wherein the green article is cured in conditions havingbetween 35% relative humidity and 45% relative humidity.
 20. A methodaccording to claim 14, wherein the green article is fired having amoisture content in the range of 1 to 5%.
 21. A method according toclaim 14, wherein the green article is fired having a moisture contentin the range of 2 to 4%.
 22. A method according to claim 14, wherein thefiring temperature is in the range of 1000° C. to 1300° C.
 23. A methodaccording to claim 14, wherein the firing temperature is between 1100°C. and 1250° C.
 24. A method according to claim 1, wherein the greenarticle is a building brick and the firing temperature is in the rangeof 1100° C. to 1250° C.
 25. A method according to claim 14, wherein thegreen article is a building brick and the firing temperature is in therange of 1100° C. to 1250° C.
 26. A building element having a matrix ofsintered fly ash and having a compressive strength of greater than 30MPa, a modulus of rupture greater than 5 Mpa, an initial rate ofabsorption (IRA) of between 0.2 to 5 kg/m²/min and an absorptioncapacity of between 5-20%.
 27. A building element according to claim 26,wherein the building element is a building brick.
 28. A building elementaccording to claim 26, when made by a method according to claim
 1. 29. Abuilding element according to claim 27, when made by a method accordingto claims 1.